Long-term objectives. The brain serotonin (5-HT) system plays an important role in cerebrovascular and neuroendocrine control. These systems have been implicated in migraine. Migraine is a low 5-HT syndrome and attacks may be triggered by a massive release of 5-HT acting on sensitized receptors. This proposal will elucidate the association between the phenomena of decreased 5-HT neurotransmission and altered cerebrovascular and neuroendocrine responsiveness. Focus will be on 5-HT receptors recently implicated in migraine pathogenesis and/or prophylactic treatment (5-HT1A, 5-HTT, 5-HT2s and 5-HT2c). As a key activator of the hypothalamic-pituitary-adrenal (HPA) axis, the role of the brain angiotensin II (Ang II) system will be addressed. The proposal intends to shed light into the pathophysiological mechanisms of migraine, and the mechanism of action of migraine prophylactic 5-HT and Ang II drugs. Specific aims. The following hypotheses will be challenged: 1) A decreased 5-HT transmission in the brain will cause sensitization and/or up-regulation of 5-HT receptor subtypes in the cerebral vasculature and the HPA axis; treatment with a migraine prophylactic compound that target these receptors will restore 5-HT receptor function and/or expression. This may be a useful animal model for drug screening in migraine prophylaxis. 2) The response to stress, which involves sequential activation of the brain Ang II and the HPA systems, will lead to decreased brain 5-HT levels, up-regulation of 5-HT receptors, and/or amplified neuroendocrine and cerebrovascular responses to 5-HT receptor activation; treatment with an inhibitor of Ang II synthesis will restore serotonergic function. Design. 1) Cerebrovascular and neuroendocrine responses to 5-HT agonists, and expression of 5-HT receptors in the cerebral vasculature and the HPA axis, will be determined in control and 5-HT-depleted Wistar rats. It will be determined whether chronic treatment with a migraine prophylactic 5-HT antagonist restore 5-HT receptor function and/or expression. 2) Brain 5-HT content, expression of 5-HT receptors in the HPA axis, and neuroendocrine and cerebrovascular responses will be determined in control and stressed (acute and chronic isolation and restraint) Wistar rats. Reversal of stress-induced changes in these variables will be attempted by chronic treatment with the angiotensin-converting enzyme inhibitor, lisinopril (i.e. a migraine prophylactic agent). In vivo cerebrovascular reactivity will be assessed by laser- Doppler flowmetry (cortical blood flow) and the 4-iodo-[N-methyl-14C]-antipyrine method (regional cerebral blood flow). In vitro cerebrovascular reactivity will be analyzed with an arteriographic chamber system. The hormonal response (ACTH, corticosterone and prolactin) will be measured by radioimmuno assay in blood samples and 5-HT receptor expression will be determined by quantitative receptor autoradiography in tissue sections. 5-HT content will be measured by HPLC in brain homogenates.